This addresses comments of @gschlager to #14531:

• extracts the common logic of overriding usernames into separate method
• fixes usernames comparison when checking if a username should be overridden. We should unicode normalize a new username before comparison, not only down case it. Note also that this logic was extracted into a separate method:
  

  discourse/app/models/user.rb

  Lines 1441 to 1443 in 78cfaa6

  	def username_equals_to?(another_username)
  	username_lower == User.normalize_username(another_username)
  	end

• adds using of UsernameChanger instead of just setting a new username in order to update quotes and mentions when renaming (which addresses this complaint https://meta.discourse.org/t/updating-mentions-when-username-is-changed-via-sso/202992)